There is a need for accurate registration of images in color printing using the cylinder (roll) rotogravure process. For example, consumer products are quite often provided to customers packaged in a paperboard (cardboard) carton or a container made of some other flexible or rigid material. The package may have a picture illustrating the product, the use of the product, or some other image intended to promote the sale of the product. In particular, many cosmetic products, such as hair dye kits (hair color kits) and hair shampoo, are packaged in paperboard (cardboard) cartons and other containers. The cartons are often printed with a color picture depicting a model having the color of hair which the hair dye is designed to produce. Such cartons are printed using a sheet-fed printing process which may be expensive, and may pose inventory problems.
It is commercially important that the picture of the model be both pleasing and accurate. Some of the most common problems in achieving a pleasing and accurate picture are the following:
The skin tones of the model are not natural, but instead are off-color, as compared to the lifelike image utilized as a reference ("matchprint"), i.e., the skin tones are too red or uneven.
The hair color is not an accurate representation of the color which is sought to be produced by the hair dye kit.
There is a lack of consistency in the pictures, so that a picture on a carton from one printing run, when displayed on a store shelf next to a carton from the same or a different printing run, will not match in appearance.
Poor image quality quite understandably reflects negatively on the consumer's purchasing decision, especially in the hair color field where the consumer should see an accurate rendition of the color sought to be obtained by treatment of the hair.
In order to obtain an acceptable printed image on hair color kit cartons, it has been necessary to print using a sheet printing process and to print on the highest quality of bleached white paperboard called "SBS" (Solid Bleached Sulphate) paperboard. SBS paperboard, however, is generally expensive compared to lower grades of paperboard, such as paperboard using reprocessed paper, for example, "clay coated news". In addition, in some countries SBS paperboard is not available and a lower quality paperboard must be used, resulting in a lower quality image.
The SBS paperboard used in the prior art manufacture of cosmetic cartons, especially hair color kit cartons, that require high quality images, is provided to the printer in the form of sheets (e.g., 3 ft. by 6 ft. and 20 pound paperboard). They are printed using a sheet-fed process. A large quantity (500 to 2000 sheets) of such SBS paperboard sheets are loaded in a magazine upstream of the printer and are fed individually into the printer, which comprises rollers for conveying the sheet and printing cylinders for depositing the process inks and the line inks. The high quality of the printed image, required for these cosmetic products cartons, limits significantly the printing speed for the SBS sheets (typically from about 30 to about 50 sheets per minute). The SBS paperboard must be fed into the printer in a certain orientation, due to grain direction of the paperboard, and, after being printed, is die-cut to form the carton blank. This may limit the designer's ability to maximize the use of the sheet for the carton and thereby minimize the waste areas between cartons.
The printed carton blanks are shipped flat to various plants, worldwide, and held in inventory. When needed, the flat carton blanks are folded into cartons, and the product is placed inside. There may be over 40 shades of hair dye in one product line. A manufacturer may have over 10 product lines and thus ma have to manufacture and inventory over 400 different carton types, i.e., over 400 different pictures and texts. Consequently, over 400 different carton types must be kept in inventory and shipped to numerous plants, many of which may be in foreign countries. It is quite costly to manufacture, ship and maintain a complete inventory of hundreds of different carton types in a worldwide distribution network. In addition, the cartons may, over time, absorb moisture and become unusable.
It is very expensive to print "short runs", i.e., of 10,000 to 100,000 cartons. But since there are so many carton types, many short runs are required to be printed. Consequently, the cost of SBS paperboard cartons may be a significant portion of the manufacturing cost of a product.
Printed labels are often used for packaging on which it may be difficult or costly to print directly on the package. For example, beer, wine and other glass bottles use printed labels, which are adhered to the bottles, as it is difficult to print in color on glass. Printed adhesive labels are also used on paperboard cartons, plastic bottles and other containers. However, in these cases the quality of the printed image is not critical, i.e., there is no attempt to obtain an accurate reproduction of a hair tone which is uniform from one printing batch to the next or within batches.
Labels are currently, and generally, of various types. One label type is a sheet of paper which is printed on one face. An adhesive may be applied to the unprinted face of the label in a label-applying machine just before the label is pressed against a container. Another label type uses a single layer web, without a supporting web, that is printed to form a plurality of images (labels). The labels are cut from the web in a separate operation and applied to the container by adhesive. Another type of label system, called "pressure sensitive labels", consists of a bottom supporting web (release liner), generally of a low grade of paper stock, plastic film or hybrid material, and the label itself, which may be of a high grade of paper stock. The label is part of a top layer of the web. The label is printed on one face (front face) and has a pressure sensitive adhesive (permanent or removable) on its opposite face (back face). The pressure sensitive adhesive retains the label on the supporting web and a release coating on the supporting web permits the label to be removed. The label is removed from the supporting web, generally by machine, and then pressed on the product or container, generally by the same machine.
The term, as used herein, of "pressure sensitive label" refers to a label removably held to a supporting web by a pressure sensitive adhesive and which is capable of being printed, removed from the supporting web, and pressed onto a container. The printing of such pressure sensitive labels may be performed by various printing methods. The preferred method is rotogravure printing on a continuous web using the conventional four-color process, with possibly additional color inks. The four-color printing process uses four printing cylinders which are inked, respectively, with process (transparent) yellow, process magenta, process cyan and process black ink. Line colors can be added using other colors of printing inks, for example, gold and line black inks. Rotogravure is a roll (web-fed) process in which cells are etched to form cells on the surface of the printing cylinder to form an image area. The cylinder surface is flooded with ink, the image area retaining ink within the cells, scraped clean of excess ink, i.e., using a doctor blade, then directly pressed on the paper, transferring the ink to the paper. The etching is in the form of tiny cells, typically 22,500 cells/in.sup.2 (for example, 250 cells per line of inch); however, more cells per inch may be used and the cells may vary in depth to provide a variation in ink capacity. The web-fed printing process is at high speed, at least 10,000 linear feet per hour. The preferred web-printing process is rotogravure; however, photogravure and off-set printing, as well as other types of cylinder (roll) printing, are within the terms "web-fed printing" and "cylinder printing".
The use of a continuous web, instead of a flat sheet, is a relatively economical method in which the product is a roll of pressure sensitive labels including a supporting web. The roll of labels is adapted to be used in an automatic label-applying machine which applies the labels to a die-cut carton or other container. In the process of the present invention, it is critical that the print from each cylinder be exactly aligned (registered) on the label to obtain an accurate and life-like image. Any misregistration may result in an image whose color is inaccurate or which is fuzzy in detail.
Labels with pictures depicting a model for cosmetic products, especially pictures showing a head of hair for hair color kits, have not been used because of the difficulty of obtaining a high-quality and color accurate image when printing labels carried on a web at high speed (more than 10,000 linear feet per hour). If there is any misregistration, even by a hair's width, the shade of the model's hair may be inaccurate or the picture of the head of hair may be blurred.
The web is processed through a printer by take-up rollers which pull the web from a supply roll. Any variation in the printing conditions, including variations in temperature, humidity, pull-roll speed and/or tension, may cause slight movement of the web away from its intended path of travel. Such slight movements of the web, minutely distorting the images, can be cumulative through the course of processing an entire web, leading to visible degradations in the printed labels. For example, if a run of 10,000 labels is to be printed and the minute misregistrations are cumulative, the image on the first label at the beginning of the run will not match the images from the middle or end of the run. If the labels are applied to cartons, and the non-matching labels end up on a store shelf side-by-side, the differences in hair tone and/or color may be noticeable to a consumer. That consumer may be confused as to the actual hair color which she seeks or may feel that the product is inferior because its packaging is non-uniform.
Because hair dye kits, in some cases, are impulse items, a potential customer who feels even subliminally uneasy because of a sub-standard image on the packaging may purchase a competitive product.
It is known, in the prior art of rotogravure printing, to print a set of square borders on the margin of the top face of the web and then print different color squares within the borders. Each square is printed with one color of ink, which identifies the position (registration) of each color printing cylinder. The color square's position relative to its borders and the other squares is read by an operator and a reader (video-magnifier) and its data is entered into the printer's controlling computer. In addition, the required tension is printed on the web's margin using a bar code. The tension is adjusted based on a system of load cells connected to the controlling computer. The operator and computer adjust the take-up rolls at each cylinder, or cylinders, to attain the exact required position of the web. Generally such adjustment is performed by slight, millimeter range, movement of the rollers or cylinders in the direction of their axes. That web position control is critical for the printer's ability to accurately register each of the printing cylinders. However, misregistration may occur even when the multi-square system is used because it is difficult to immediately detect, and correct, misregistrations.
An alternative method for sensing color registration is disclosed in U.S. Pat. No. 4,546,700, incorporated by reference. In that method a series of black ring-like area marks are printed on the margin of a web, each of the marks being the same. Each color is printed as a block, over its own black mark. The extent that each black mark's hole is covered by a color block is measured, using a reflectometer, to determine that color's registration.